The present invention relates to a method of inhibiting metal sulfate scale formation in an aqueous system. More particularly, the invention is directed to the use of certain poly(amino acids) to inhibit scale formation of metal sulfates. The poly(amino acids) are preferably added with at least one inorganic phosphate to the aqueous system.
Metal sulfate scale formation is a common problem in many aqueous systems. The sulfate scale is formed in the aqueous systems, when cations, for example alkaline earth metal ions, combine with sulfate ions. Metal sulfate scale includes for example magnesium sulfate, calcium sulfate, barium sulfate, strontium sulfate, radium sulfate, iron sulfate, and manganese sulfate.
Aqueous systems where metal sulfate scale may form are for example in cooling water systems, boilers, heat exchange equipment, reverse osmosis equipment, sugar processing equipment, geothermal systems, oil and gas production operations, flash evaporators, desalination plants, paper making equipment, and steam power plants. Metal sulfate scale is particularly found in paper making equipment, desalination plants, reverse osmosis, and oil and gas production operations.
The metal sulfate scale forms on the surfaces of the aqueous systems causing such problems for example as reduced heat transfer, plugged pipes, and acceleration of corrosion. Once formed, the sulfate scale is difficult to remove. Scraping, sand blasting, chipping, or chemical removal with specially formulated cleaners may be required to remove the scale.
To prevent the formation of metal sulfate scale, scale inhibitors are added to aqueous systems.
For example, in papermaking processes, scale inhibitors are added to prevent excessive scale build-up in the equipment, such as in the headbox and related piping, and on the fourdrinier wire. The metal sulfate scale is formed from the interaction of the alkaline earth metals and sulfate ions found in components used in the paper making process. For example, barium and other alkaline earth metals are found in wood pulp, which is a primary ingredient in the paper making process. Sources of sulfate ions in the paper making process are, for example, from water or aluminum sulfate which may be intentionally added to increase retention of other additives during formation of paper.
Scale inhibitors are used in equipment to purify sea water or brackish water in desalination plants. Such equipment includes for example reverse osmosis equipment and distillation units. Because sea water or brackish water contains both sulfate ions and alkaline earth metals, as the water is purified, the metal sulfate concentrates and deposits on the equipment surfaces to form scale. The scale formation can be a serious problem causing the equipment to operate less efficiently leading to equipment downtimes for cleaning and increased operating costs.
In oil production operations, the formation of metal sulfate scale is a common and serious problem. Consequently scale inhibitors are commonly used. The metal sulfate scale, most commonly barium sulfate and calcium sulfate, typically form when alkaline earth metal ions from an aquifer or from connate water, combine with water containing sulfate ions, such as sea water or another aquifer previously unconnected to the first aquifer. The metal sulfate deposits as scale in subterranean formations or in topside equipment such as in pipes, valves, and pumps.
Scale inhibitors are used in various ways to prevent scale formation in oil production operations. For example, in what is called a "squeeze treatment", an aqueous solution, containing scale inhibitor, is forced under pressure through the wells into the subterranean formation. The scale inhibitors are believed to adsorb or precipitate onto the formation and gradually desorb or resolubilize from the formation to inhibit scale from depositing and prevent clogging of the formation with scale. The well is then periodically resqueezed when the concentration of scale inhibitor falls below an effective concentration for scale inhibition.
Scale inhibitors may also be fed into pipelines to prevent scale formation, which would impede the transport of oil. Additionally, scale inhibitors are used in secondary oil recovery operations, where pressurized water is used to recover additional oil.
Scale inhibitors which have been used in aqueous systems to inhibit metal sulfate scale formation are for example homopolymers and copolymers of acrylic acid. More recently, there has been a trend to develop scale inhibitors which have greater biodegradability. For example, U.S. Pat. No. 5,116,513 to Koskan, et al., discloses the use of a poly(amino acid), poly(aspartic acid), having a molecular weight of 1000 to 5000 as a scale inhibitor for calcium sulfate and barium sulfate. The poly(aspartic acid) is produced by the thermal condensation of aspartic acid to form polysuccinimide. The polysuccinimide is then hydrolyzed to form poly(aspartic acid) which preferably is greater than 50 percent in .beta.-form and less than 50 percent in .alpha.-form. The poly(aspartic acid) is disclosed to be biodegradable.
The problem addressed by the present invention is to provide additives which improve the performance of poly(amino acids) to inhibit metal sulfate scale formation. The present invention also seeks to provide certain poly(amino acids) which more effectively inhibit metal sulfate scale formation.
We have found that adding one or more inorganic phosphates and one or more poly(amino acids) to an aqueous system to inhibit metal sulfate scale formation, is more effective than what would be expected by adding either the phosphates or poly(amino acids) alone to the aqueous system.